Variable-width pulse integrator



June 3, 1969 A. L. NEWCOMB, JR 3, 48,290

VARIABLE-WIDTH PULSE INTEGRATOR Filed March 15, 1966 INVENTOR ARTHUR L.NEWCOMB, JR.

United States Patent 3,448,290 VARIABLE-WIDTH PULSE INTEGRATOR Arthur L.Newcomb, Jr., Newport News, Va., assignor to the United States ofAmerica as represented by the National Aeronautics and SpaceAdministration Filed Mar. 15, 1966, Ser. No. 536,216 Int. Cl. H03k 5/20US. Cl. 307-234 9 Claims ABSTRACT OF THE DISCLOSURE A solid state devicefor converting variable-width pulses into an analog voltage. Twocapacitors are alternately charged with a constant current during thedurations of said variable-width pulses. The two capacitors aredischarged during the times that they are not being discharged. Thecharged capacitors are selectively applied to the output of the deviceto provide the desired output analog voltage.

The invention described herein was made by an employee of the UnitedStates Government and may be manufactured and used by or for theGovernment for governmental purposes without the payment of anyroyalties thereon or therefor.

The invention relates generally to an integrator and more specificallyconcerns a solid state integrator for converting variable width pulsesinto an analog voltage.

In an electronic instrument Where pulses are generated whose widths areproportional to some desirable parameter, a device is needed forproducing an analog voltage having a magnitude indicative of the pulsewidths. The infrared horizon scanner disclosed in US. Patent No.3,038,077 is an instrument that creates a need for such a device. Theoutput of this instrument is pulses whose widths are directlyproportional to the attitude error of a space vehicle with respect to aplanetary body. Integration of these pulses, for use with an attitudecontrol system, is exceedingly difiicult since the duty cycle (timebetween pulses versus pulse width) is extremely small at the point ofinterest (where attitude error approaches zero).

In conventional R-C integrators, duty cycle is a parameter whichdirectly effects voltage fluctuations, or ripple. This undesirableripple approaches the maximum at zero duty cycle.

Another prior art integrator uses two capacitors which are alternatelycharged and read. While one capacitor is being charged, the other isbeing read through a high input impedance circuit. The charge and readpath are switched by a DPDT relay which is operated at the end of eachpulse. Since relays have a limited operating lifetime, draw appreciablepower, and cannot be switched in much less than two milliseconds, anintegrator that uses them is undesirable for space applications.

It is therefore an object of this invention to provide a solid stateintegrating circuit for converting variablewidth pulses to analogvoltages.

Another object of this invention is to provide an integrator that has avery low ripple content in the low duty cycle range.

A further object of this invention is to provide an integrator forconverting variable-width pulses into an analog voltage in which thereis no time lag, or hysteresis, between the input and the output of theintegrator.

Still another object of the invention is to provide an integrator, forconverting pulse widths into an analog voltage, which has a low powerconsumption and a long lifetime for use on a spacecraft.

A still further object of this invention is to provide a 'ice pulsewidth integrator which has a low ripple content in its output over theentire range of the integrator.

The present invention is an integrator which operates similar to thetwo-capacitor integrator mentioned above. The invention consistsessentially of two electrical circuits including a capacitor in eachcircuit. The input pulses are applied to a constant current networkwhich produces a constant current in each of said two electricalcircuits during the durations of the input pulses. The magnitude of thisconstant current is independent of the amplitudes of the input pulses. Abistable multivibrator in combina tion with two diodes provide means foralternately charging the two capacitors with the constant current in thetwo circuits. The bistable multivibrator in combination with two othercapacitors and two silicon controlled switches provide means foralternately discharging the two capacitors. An OR gate is connected tothe two capacitors so that the greater voltage across the two capacitorsis applied to the output of the integrator. The amplitude of this outputvoltage is proportional to the widths of the input pulses.

Other objects and advantages of this invention will further becomeapparent hereinafter and in the single drawing which is a schematicdiagram of the invention.

In describing the preferred embodiment of the invention illustrated inthe drawing, specific terminology will be resorted to for the sake ofclarity. However, it is not intended to be limited to the specific termsso selected, and it is to be understood that each specific term includesall technical equivalents which operate in a similar manner toaccomplish a similar purpose.

Turning now to the specific embodiment of the invention selected forillustration in the drawing, the numbers 11 and 12 designate the inputterminals to the integrator. A resistor 13 and a resistor 14 areconnected in series between terminals 11 and 12. The junction ofresistors 13 and 14 is connected to the base of an NPN transistor 15whose emitter is connected directly to ground. A resistor 16 and aresistor 17 are connected in series between a power source and thecollector of transistor 15. The junction of resistors 16 and 17 isconnected to the base of a P-NP transistor 18 whose emitter is connecteddirectly to the power source. The collector of transistor 18 isconnected to the collector of an NPN transistor 19, and to a bistablernultivibrator 21 including NPN transistors 22 and 23. The collector oftransistor 18 is also connected to the base of transistor 19 through aresistor 24 and a diode 25, and through a resistor 26 and a diode 27.The junction of resistor 24 and diode 25 is connected to the emitter oftransistor 19 through a Zener diode 28 and a variable resistor 29; andthe junction of resistor 26 and diode 27 is connected to the emitter oftransistor 19 through a Zener diode 3t and a variable resistor 31.

The junction of Zener diode 28 and variable resistor 29 is connectedthrough a diode 32 to the collector of transistor 23 and also thejunction is connected through diodes 33 and 34 to the base of an NPNtransistor 35. The junction of Zener diode and variable resistor 31 isconnected through a diode 36 to the collector of transistor 22 and alsothe junction is connected through diodes 37 and 38 to the base oftransistor 35. Zener diodes 28 and 30 and diodes 33 and 37 should bematched. A capacitor 39 and a silicon controlled switch 40 are eachconnected from the junction of diodes 33 and 34 to ground. A capacitor41 and a silicon controlled switch 42 are each connected from thejunction of diodes 37 and 38 to ground. Silicon controlled switches 40and 42 each has a high impedance except when a positive voltage isapplied to its terminal. Then it has a low impedance. A capacitor 43 isconnected between the collector of transistor 23 and the controlterminal or" silicon controlled switch 40; and a capacitor 44 isconnected between the collector of transistor 22 and the controlterminal of silicon controlled switch 42. The collector of transistor 35is connected directly to the power source. Output terminals 46 and 47are connected across a resistor 48 and a resistor 49, the junction ofwhich is connected to the emitter of transistor 35.

Multivibrator 21 is a conventional type multivibrator except that itcontains circuitry to enable it to be triggered on the trailing edge ofa pulse. This circuitry is capacitors 50 and 51, resistors 52 and 53,and diodes 54 and 55. Suppose that transistor 23 is conducting andtransistor 22 is not conducting, and a positive pulse is applied to thejunction of capacitors 50 and 51. The circuit through capacitor 50,resistor 52 and transistor 23 to ground forms a differentiating circuitwhich produces a sharp negative pulse at the trailing edge of thepositive pulse. This sharp negative pulse is applied through diode 54 tothe base of transistor 23 to out OK transistor 23 and cause transistor22 to become conductive. The trailing edge of the next positive pulseproduces a sharp negative pulse at the junction of capacitor 51 andresistor 53 which is applied through diode 55 to the base of transistor22 to cut it off and cause transistor 23 to again become conductive.

The circuitry consisting of transistor 19, Zener diodes 28 and 30 andvariable resistors 29 and 31 is a constant current circuit. Resistors 24and 26 are Zener-bias resistors. This circuitry insures the linearity ofthe output voltage with respect to the width of the input pulses. Eachtime an input pulse is applied to input terminals 11 and 12 a constantcurrent is produced at the junction of Zener diode 28 and variableresistor 29 and at the junction of Zener diode 30 and variable resistor31. These currents are present throughout the duration of each pulse.The magnitudes of the currents can be regulated by resistors 29 and 31.

Diode 33 prevents capacitor 39 from discharging through resistors 29 and20 to ground and diode 37 prevents capacitor 41 from discharging throughresistors 31 and 20 to ground. Diodes 34 and 38 form an OR gate whichinsures that the higher of the two voltages across capacitors 39 and 41is applied to the base of transistor 35. Transistor 35 is connected inan emitter-follower circuit to provide a high impedance to preventdischarging of capacitors 39 and 41, while maintaining a relatively lowoutput impedance.

In operation when a first input pulse is applied to input terminals 11and 12, it causes the base of transistor 15 to become positive whichcauses the transistor to conduct and produce a voltage drop acrossresistor 16. The only requirement made of the input pulse is that it hasa fall time sufliciently fast to trigger multivibrator 21 and anamplitude great enough to saturate transistor 15. The voltage dropacross resistor 16 causes transistor 18 to conduct and produce apositive voltage at its collector. This positive voltage is applied tothe constant current network consisting of transistor 19, Zener diodes28 and 30 and variable resistors 29' and 31. Consequently, currentsappear at both the junction of Zener diode 28 and variable resistor 29,and the junction of diode 30 and variable resistor 31. At the time thefirst input pulse is applied to the input terminals one of thetransistors in multivibrator 21 is conducting. Let us assume that theone that is conducting is transistor 23. Then the collector oftransistor 23 is approximately at ground potential which allows thecurrent at the junction of Zener diode 28 and variable resistor 29 toflow through diode 32 to ground. At the same time, the collector oftransistor 22 is at a positive potential which biases diode 36 so thatit has a very high impedance. Thus, the current at the junction of Zenerdiode 30 and variable resistor 31 flows through diode 37 into capacitor41, thereby charging the capacitor. At the end of this first pulse, avoltage appears across the capacitor 41 that is proportional to theduration of the pulse. This voltage is applied through diode 38 to theba e of t a i tor 35 thereby producing an output voltage acrossterminals 46 and 47. The amplitude of this output voltage isproportional to the width of the input pulse.

The pulse at the collector of transistor 18 is applied to the junctionof capacitors 50 and 51. Consequently, at the conclusion of this firstpulse, a sharp negative pulse is produced between the junction ofcapacitor 50 and resistor 52. This pulse is applied through diode 54 tothe base of transistor 23 which causes transistor 23 to cut off. Whentransistor 23 is cut off, transistor 22 automatically becomesconductive. When the second input pulse is applied to terminals 11 and12, the collector of transistor 22 is connected to ground, allowing thecurrent at the junction of Zener diode 30 and variable resistor 31 toflow through diode 36 to ground. Diode 37 prevents capacitor 41 fromdischarging through diode 36. Since transistor 23 is now cut oii, thecollector of transistor 23 is at a high positive potential which biasesdiode 32 so that the current at the junction of Zener diodes 28 andvariable resistor 29 will flow through diode 33 into capacitor 39 andcharge it. At the termination of the second pulse a sharp negative pulseis produced at the junction of capacitor 51 and resistor 53 which isapplied through diode 55 to the base of transistor 22 which causestransistor 22 to cut off. When transistor 22 is cut oil, its collectorvoltage becomes positive and transistor 23 automatically becomesconductive. This rise in voltage on the collector of transistor 22 isapplied through capacitor 44 to the control terminal of siliconcontrolled switch 42 which lowers the impedance across the switch. Thisallows capacitor 41 to discharge through the switch to ground. At thistime, the voltage across capacitor 39 is proportional to the width ofthe second input pulse. This voltage across capacitor 39 is appliedthrough diode 34 to the base of transistor 35 which produces a voltageat output terminals 46 and 47 proportional to the width of the secondinput pulse.

When the third input pulse is applied to input terminals 11 and 12 theresulting constant current at the junction of Zener diode 28 andvariable resistor 29 passes through diode 32 and transistor 23 toground. The current at the junction of Zener diode 30 and variableresistor 31 passes through diode 37 and charges capacitor 41. Thisprocess continues for each additional input pulse. It should be notedthat from the termination of the first pulse to the termination of thesecond pulse the voltage across output terminals 46 and 47 isproportional to the width of the first pulse; and from the terminationof the second pulse to the termination of the third pulse the outputvoltage is proportional to the width of the second pulse. As can be seenthe output of the integrator is always proportional to the width of thelast input pulse applied to it. This output will be present from thetermination of the last input pulse until the termination of the nextinput pulse. The output of the integrator can be considered as anintegral function of the input on a single pulse basis, or as an RMS(root mean square) equivalent of a series of evenly spaced pulses.

The advantages of the integrator that constitutes this invention arenumerous. Its output is linearly proportional to the width of the inputpulses; it is operable for extremely small duty cycles; the output has avery low ripple content over the entire range of the integrator; it hasa low power consumption and a long lifetime making it desirable forspace applications; and there is no time lag, or hysteresis, between itsinput and output.

It is to be understood that the form of the invention herewith shown anddescribed is to be taken as a preferred embodiment. Various changes maybe made in the shape, size, and arrangement of parts. For example,equivalent elements may be substituted for those illustrated anddescribed herein, parts may be reversed, and certain features of theinvention may be utilized independently of the use of other features,all Without departing from the spirit or scope of the invention asdefined in the subjoined claims. Although the integrator is intended forindicating position in a spacecraft attitude sensing system, it couldprove useful in any similar application where low duty cycles might beencountered. For example, it may convert voice modulated variablewidthpulses or telemetered variable-width pulse intelligence to analogvoltage changes over narrow bandwidth channels.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. A variable-width pulse integrator comprising: means receiving theinput pulses to be integrated, for producing a constant current duringthe durations of said input pulses that is independent of the amplitudesof the input pulses; a first and -a second capacitor means; means foralternately charging said first and second capacitor means with saidconstant current during the durations of said input pulses; means fordischarging each capacitor during the time that it is not being charged;and means for applying the larger of the voltages across said first andsecond capacitor means to the output of said integrator whereby theamplitude of the voltage at the output of said integrator isproportional to said input pulses.

2. A variable-width pulse integrator according to claim 1 wherein saidmeans for alternately charging said first and second capacitor meansincludes a bistable multivibrator.

3. A variable-width pulse integrator according to claim 1 wherein saidmeans for applying the larger of the voltages across said first andsecond capacitor means to the output of said integrator includes an ORgate.

4. A variable-width pulse integrator comprising: a first electricalcircuit including a first capacitor; a second electrical circuitincluding a second capacitor; means, receiving the input pulses to beintegrated, for producing a constant current in said first and secondelectrical circuits during the durations of said input pulses; abistable multivibrator which receives said input pulses and changesstate after receiving each input pulse; means including saidmultivibrator for steering the constant current in said first electricalcircuit into said first capacitor to charge it while said multivibratoris in one of its two states and for steering the constant current insaid second electrical circuit into said second capacitor to charge itwhile said multivibrator is in the other of its two states;

means including said multivibrator for discharging said second capacitorwhile said multivibrator is in its said one state and for dischargingsaid first capacitor while said multivibrator is in its said otherstate; and means for applying the larger of the two voltages across saidfirst and second capacitors to the output of said integrator whereby theamplitude of the voltage output of said integrator is directlyproportional to the widths of the input pulses.

5. A variable-width pulse integrator according to claim 4 wherein saidbistable multivibrator includes means that causes it to change state atthe terminal end of each input pulse.

6. A variable-width pulse integrator according to claim 4 wherein saidmeans for steering the constant current in said first and secondelectrical circuits into said first and second capacitors includes twodiodes.

7. A variable-width pulse integrator according to claim 4 wherein saidmeans for applying the larger of the two voltages across said first andsecond capacitors to the output of said integrator includes an OR gate.

8. A variable-width pulse integrator according to claim 4 wherein saidmeans for discharging said first and second capacitors includes a firstswitch connected across said first capacitor and a second switchconnected across said second capacitor with both switches beingcontrolled by said multivibrator.

9. A variable-width pulse integrator according to claim 8 wherein saidfirst and second switches are silicon controlled switches.

References Cited UNITED STATES PATENTS 2,888,579 5/1959 Wanlass 307-2183,076,933 2/1963 Negrete 235-l83 3,099,001 7/1963 Syptak 307246 ARTHURGAUSS, Primary Examiner. B. P. DAVIS, Assistant Examiner.

US. Cl. X.R.

